1. Field of the Invention
The present invention relates to a magneto-optical medium for achieving high density recording and a reproducing device for the magneto-optical medium.
2. Description of Related Art
On an erasable magneto-optical disk, information is recorded as follows: A magneto-optical recording film is heated through irradiation with a laser beam. As a result, a mark indicating the direction of magnetization is formed on the heated portion so that the direction corresponds to that of the external magnetic field in accordance with the information to be recorded. In reproducing the recorded information, the track of such marks is irradiated with a laser beam so as to utilize the Kerr effect that the plane of polarization of the reflected light is rotated in accordance with the direction of magnetization. In such a conventional magneto-optical recording/reproducing system, the entire spot area of the laser beam on the magneto-optical disk is used as an area for detecting a signal to be reproduced. Therefore, the reproducible linear recording density is determined by the spot diameter of the irradiating laser beam.
A magneto-optical disk has been regarded as a leading memory for storing data increasing in the recent rapidly developing multimedia, and has been desired to have a larger storage capacity. The reproducible linear recording density, however, is determined by the spot diameter of the laser beam as described above, and the spot diameter is limited by optical problems of a laser beam source and the like. Thus, it was difficult to achieve high density recording.
In order to achieve high density recording, a magnetically induced super resolution medium (hereinafter referred to as the MSR medium) and the recording/reproducing system for the MSR medium are proposed. In this recording/reproducing system, a disk bearing a lamination of a plurality of magnetic films respectively having different magnetic characteristics depending upon temperature is used to read data from a part of the spot area of the laser beam. Therefore, even when a mark is smaller than the area determined by the spot diameter of the laser beam, the data can be steadily read. The MSR medium and the recording/reproducing system for the MSR medium will now be described.
FIG. 1 shows the structure of the MSR medium and the positional relationship between a mark and the spot area of the laser beam on the recording track disclosed in Jpn. J. Appl. Phys. Vol. 31 (1992) (pp. 568-575 Part 1, No. 2B, February 1992). A recording film 1 formed on a transparent substrate (not shown) includes three layers, i.e., a reproducing layer 2, a switching layer 3 and a recording layer 4 in this order from the substrate. Data is recorded on the recording layer 4 in accordance with the direction of magnetization, and marks 6 are formed on a recording track 5 with a narrower pitch therebetween than the spot diameter of a laser beam to be used for irradiation. With regard to a magneto-optical disk, the power of a laser beam for recording can be controlled so that an area to be heated up to a temperature over a Curie temperature (Tc) is made smaller than the spot. Therefore, it is not difficult to form small marks.
The reproducing operation by using the MSR medium is as follows: At room temperature, the direction of magnetization in the reproducing layer 2 coincides with that in the recording layer 4 due to the exchange coupling force there-between through the switching layer 3. In an area where the temperature is raised over the Tc of the switching layer 3 by the laser beam irradiation for reproducing (herein referred to as a high temperature area), however, the exchange coupling force with the recording layer 4 is lost. Therefore, the direction of magnetization in such a high temperature area in the reproducing layer 2 coincides with the direction of externally applied reproducing magnetic field (Hr). As a result, the high temperature area works as a mask for masking the marks therein, and data are read from a low temperature area (i.e., an area where the temperature is not over the Tc) on the recording layer 4. In this manner, the marks can be read from an area smaller than the spot diametei of the laser beam for reproducing, and the obtained resolution is substantially as high as that obtained when a laser beam for reproducing has a smaller light spot.
FIG. 2 is a graph showing the reproducing signal characteristics in the MSR medium and a conventional magneto-optical medium. The MSR medium exhibits excellent characteristics (C/N) even when the linear recording density is increased and a mark is smaller.
The above-mentioned method, that is, the method in which a high temperature area in the laser spot area is masked and a mark in a low temperature area is read, is designated as a front aperture detection (FAD) method. Another method, in which a low temperature area in the laser spot area is masked and a mark in a high temperature area is read, is known as a rear aperture detection (RAD) method, which also attains high resolution reproducing.
The RAD can he realized by a magneto-optical disk including two layers: a reproducing layer and a recording layer. An initializing magnetic field is applied just before the irradiation of the reproducing laser beam is focused upon a ma to be read causing the direction of magnetization in the reproducing layer to coincide with that of the initializing magnetic field. At this point, the marks in the recording layer remain unchanged. Just after the application of the initializing magnetic field, the reproducing layer works as a mask for masking the data on the recording layer. Subsequently, the irradiation of the reproducing laser beam raises the temperature of the reproducing layer working as the mask at the position of the mark to be read. When the exchange coupling force between the reproducing layer and the recording layer becomes larger than the coercive force of the reproducing layer as a result of the temperature rise, the direction of magnetization in the recording layer is transferred. In other words, the reproducing layer is unmasked at a high temperature area, from which the mark is read.
On a magneto-optical disk, pre-formatted information such as ID signals and ROM data are recorded as well as magneto-optically recorded general information. It is preferable that a pre-formatted recording area for the pre-formatted information and a magneto-optical recording area for the general information have the same linear recording density because the data in both recording areas can be reproduced by using a common synchronous clock generator and a common data discriminator. On the pre-formatted recording area, however, an irregularity (such as a pit) is formed (for generating an emboss signal) through a press process. Therefore, the masking effect as in the MSR medium cannot be obtained in reproducing the data in the pre-formatted recording area. Thus, in prior art devices, it was a waste of disk space to allow these recording areas to have the same linear recording density.
Japanese Patent Application Laid-Open No. 4-259941 (1992) discloses a method for recording sector management information in a pre-formatted recording area at a lower linear recording density than that for recording general information. In such a case, the sector management information can be reproduced although the super resolution effect cannot be attained.
When this method is used for a partial ROM disk, in which data of several megabytes to several tens megabytes is previously recorded as ROM data and the magneto-optical recording area is provided as a RAM area to be freely used by a user, the ROM data area is required to be large because the linear recording density of the ROM data is low. This causes a problem because the RAM area for a user becomes extremely small.